The present invention relates generally to stability control systems for vehicles, and more particularly relates to methods for estimating the longitudinal velocity of a vehicle for a stability control system
Stability control systems for motor vehicles (also known as dynamic stability programs or xe2x80x9cDSPxe2x80x9d systems) typically utilize a controller to process information from various sensors throughout the vehicle to ensure that the vehicle remains dynamically stable while undergoing a turning maneuver. That is, factors such as yaw rate and yaw angle or roll rate and roll angle are monitored to ensure they do not exceed certain threshholds. The system is coupled with the vehicle braking system and a drivetrain to restore dynamic stability when the threshholds are reached. These control systems usually estimate a longitudinal velocity of the vehicle for these stability calculations.
Current methods for estimating the longitudinal velocity of a vehicle are based upon a simple general principle. Basically, this principle states that when the vehicle is accelerating, the speed of the slowest wheel (which is generally not slipping) is utilized to estimate the vehicle""s longitudinal velocity. When the vehicle is decelerating, the velocity of the fastest wheel (which is generally not slipping) is utilized to estimate the vehicle""s longitudinal velocity.
Typically a vehicle includes a speed sensor at each wheel of the vehicle. Accordingly, in a typical four-wheeled vehicle, there will be a front left tire sensor (F.L.T. sensor), a front right tire sensor (F.R.T sensor), a rear left tire sensor (R.L.T. sensor) and a rear right tire sensor (R.R.T. sensor). The current methods will first determine whether the vehicle is accelerating or decelerating. This can be accomplished in numerous ways as is known in the art, such as by monitoring the acceleration and braking pedals, or by monitoring the powertrain system. Then, the speed of either the fastest or slowest wheel is used to estimate the longitudinal velocity.
The present invention provides an improvement to these existing systems and the general principle by increasing the accuracy of the estimation of longitudinal velocity during all driving conditions.
The present invention provides a method for estimating the longitudinal velocity of a vehicle with an increased accuracy during all driving conditions, including situations of high lateral acceleration. Generally, the method comprises the steps of determining whether the vehicle has a lateral acceleration greater than a predetermined value, and then setting the estimate of longitudinal velocity based on the velocity of the wheel having the highest normal force when the vehicle does have a lateral acceleration greater than the predetermined value. The predetermined value is preferably set to reflect the vehicle being in a curve driving situation.
The method may further comprise the step of determining whether the vehicle has had a lateral acceleration greater than the predetermined value within a predetermined amount of time. Preferably, when the vehicle is not currently, but has had a lateral acceleration greater than the predetermined value within the predetermined amount of time, (i.e., coming out of a turn, a slalom situation or lane changing situation), the estimate of longitudinal velocity is set based on an integration of the vehicle""s longitudinal acceleration over time. When the vehicle does not and has not had a lateral acceleration greater than the predetermined value within the predetermined amount of time, the estimate of longitudinal velocity is set based on the fastest wheel of the vehicle when the vehicle is decelerating, and based on the slowest wheel when the vehicle is accelerating, according to the general principle.